Are fish in a shoal all the same age?

Asked by: Leslie Wise, London

In general, yes. Swirling, spiralling shoals are usually made up of fish of the same species and same size, and hence age. This is partly to confuse predators. Being a similar size, shape and colour makes it difficult for a hunter – a seal, dolphin or bigger fish – to make out and target a single prey fish. Other benefits of forming shoals include swimming efficiency and finding food. Roughly half of all fish species form shoals at some point and one in four species, including sardines, herring and anchovies, live permanently in shoals and get agitated when they’re on their own.

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